Hello Sam,
welcome at Stridaforum 
I think to have a few ideas, but, according to the complexity of your questions I’ll need some time please…going to edit the post, plz click reload here and there…
Edit1:
Well…first; if it is really a fake one I’d recommend not to use it - usually are these bikes dangerous.
As a bike mechanic you will be familiar with worst China quality…
Btw; pics could help identifying.
- Plastic bottom bracket means that could be the cheaper Strida LT version - does it have plastic wheels?
There is at least one (private) supplier of the metal excenter in Europe, but anyway I’ll edit official links later (once found).
However - you should know that you can trust Bill of Strida Canada for 100%, and shipments from Canada or Korea are often faster than trying to order parts in Europe!
That may sound like a bad joke; actually it’s the sad truth…
Technical hint for the bottom bracket/excenter: Both fixations have to be tight; the big groove nut AND the kickstand bolt, one fixation alone can’t withstand the generated torque while heavy pedaling!
2.There is a lot of play at the frame locking, that is normal.
Please consider that the bottom frame tube, under load, will be pressed forward/down - it can’t unlock that easy.
- Maybe the wrong saddle?
Not all saddles fit the Strida molding, mainly it is the distance from rails to nose, just look here:
Saddle pointing downwards
Edit2:
4. Most likely rust between axles and inner bearing rings at both wheels.
At the rear wheel you have to unmount also the snubber (part nr.: 389) and its bolt/washer.
- Just click below:
Strida Ball Joint Socket Explained
Edit3: Here’s an official European link to the desired part;
http://stridaroma.it/shop/it/products/?q=blocco+130+single+speed
please note therefore also the message at your private mailbox 
Best regards,
Chris